1. Field of the Invention
This invention relates generally to the field of marine seismic exploration utilizing seismic signal sources deployed from a geophysical exploration vessel moving along a shot track. More particularly the invention concerns a system and method for firing arrays of such sources so as to simultaneously generate two or more profiles with each pass along such track.
2. Prior Art
Typically, in conventional two-dimensional seismic surveys a seismic source is discharged at shot points evenly spaced along a linear ship track. The source is typically composed of a large number of individual air guns of differing sizes deployed in linear sub-arrays each composed of five or more guns. Two or more such sub-arrays are operationally combined to form a single complete source array.
If a three-dimensional survey is to be undertaken, the sources are discharged at points along an areal grid, usually by traversing the survey along parallel lines, collecting a single profile of shot points during each pass. However, it is evident that considerable saving in ship time and thus survey costs can be achieved by collecting multiple profiles during each pass.
The standard technique now employed for collecting such multiple profiles is to deploy a separate source array for each profile to be acquired. The separate arrays are positioned relative to the seismic hydrophone streamer so that the profiles are offset perpendicularly from the ship track following parallel paths or grid lines. While theorically this technique may be used to collect more than two profiles, the current industry limit is two profiles. The reason is that survey vessels commonly can deploy only a few more sub-arrays than their standard array contains. Each time a sub-array is added more space is necessary on the towing vessel for mounting of cable reels and for handling the various electrical and air supply lines, which are usually incorporated within the towing umbilicals. Therefore to deploy separate arrays laterally displaced from each other it becomes necessary to go to smaller arrays i.e. composed of fewer sub-arrays. The result is less energy per shot and thus poorer signal to noise ratio. If three or more simultaneous parallel profiles are desired the size of an individual array may become unacceptable reduced.
It is thus a general object of this invention to provide an improved system and method for operating two or more laterally displaced marine seismic source arrays.
A more particular object of this invention is to operate such source arrays in a manner to more efficiently utilize their available energy potential.